Phones affecting airplanes is already not that much of a risk (even if indeed existing), and besides, as the article claims, the phone might still just sent a periodic ping, which is going to be practicable unnoticeable for any equipment.
In what sense could it ever be "practicable unnoticeable", much less "for any equipment"?
Conceivably it would use a different waveform with lower energy when transmitting clandestinely, but that would be insanely expensive to achieve, probably need to add a separate chip, and it wouldn't work for people who change phones regularly.
I'm reminded that a while back the Shin Bet modified a phone to have a chunk of plastic explosive, used to kill a Hamas bomb maker. It was detonated remotely with a non-phone protocol (as the phone channel was in use by the target).